1. Field of the Invention
This invention relates to a novel vaccine against edwardsiellosis which does in fact provide superior protection over commercial treatment involving feeding medicated feeds.
2. Description of the Prior Art
Edwardsiella tarda is a Gram-negative, motile, rod shaped, aquatic bacterial pathogen which is highly infectious in both warm and cold water species of fish. The bacterium is commonly encountered in channel catfish ponds, intensive tilapia culture, and eel production systems and is, therefore, a constant threat of disease. In the channel catfish (Ictalurus punctatus), it is the causative agent of Edwardsiella septicemia disease, and has been isolated from channel catfish in areas of the southeastern United States where this species is cultured. The disease also affects numerous other cultured fish species, sport fish (e.g., largemouth bass), baitfish, and aquarium fishes. Wyatt et al. (1979, Edwardsiella tarda in freshwater catfish and their environment, Applied Environmental Microbiology, 38:710–714) found that in E. tarda positive catfish ponds, this bacterium was isolated from 75% of the pond water, 64% of pond mud, and 100% of apparently healthy frog, turtle, and crayfish samples. Meyer and Bullock [1973, Edwardsiella tarda, a new pathogen of channel catfish (Ictalurus punctatus), Applied Microbiology, 25:155–156] highlighted the food safety problem of E. tarda when they reported that 88% of dressed catfish were culture-positive for E. tarda. This situation usually results in a shut down of the processing lines until they are cleaned and disinfected. Finally, Plumb (1999, Health Maintenance and Principal Microbial Diseases of Cultured Fishes, Iowa State University Press, Ames, Iowa) recommended that care be given to handling E. tarda infected fish because of the potential risk of human infection. Edwardsiellosis is also known to affect tilapia culture in both fresh and marine waters.
Medicated feed containing antibiotics is currently used to try and control this bacterial infection. However these treatments are limited in their effectiveness and most producers have discontinued their use. Prevention of edwardsiellosis disease by vaccination is an important goal and a top priority of catfish and other fish producers throughout the world. Estimated savings to these and other industries would be in excess of millions of dollars annually. The considerable economic value of tilapia (Marine Fisheries Service, Fisheries Statistics and Economic Division report, April, 2003) and significant disease losses caused by E. tarda dictate the need for an effective E. tarda vaccine.
Killed E. tarda vaccines have been experimentally produced (Gutierrez, M. A. and Miyazaki. T., 1994, Response of Japanese eels to oral challenge with Edwardsiella tarda after vaccination with formalin-killed cells or lipopolysaccharide of the bacterium, Diseases of Aquatic Organisms, 6:110–117) but are not commercially available. Based on the success of disease control by immunization with killed bacteria (i.e., bacterins) in salmonids, experimental bacterins have been developed and tested against E. tarda (Salati, F., 1988, Vaccination against Edwardsiella tarda, In: Fish Vaccination, edited by A. E. Ellis, Academic Press, London, pp. 135–151). However, no vaccine is currently available and vaccination is not practiced in the catfish industry against E. tarda, presumably because the inactivation (i.e., formalin treatment) destroys the antigen. Wolf-Watz et al. (U.S. Pat. No. 5,284,653) disclosed that a wide variety of bacteria have the potential to be genetically modified to produce a vaccine. However, no data are presented on E. tarda vaccines, only on genetically modified mutant vaccines of Vibrio anguillarum. 
We previously developed a rifampicin resistant vaccine isolate of Edwardsiella ictaluri which was effective as a vaccine against this bacterium, a causative agent of enteric septicemia of catfish (RE-33 or Intervet AQUAVAC-ESC) (Klesius, P. and Shoemaker, C.A., 1999, Development and use of modified live Edwardsiella ictaluri vaccine against enteric septicemia of catfish, In: Advances in Veterinary Medicine, Vol. 41, edited by R. D. Schultz, Academic Press, San Diego, Calif., pp. 523–537). However, this vaccine is not protective against E. tarda infection in channel catfish.